1. Field of the Disclosure
The present disclosure is related generally to the field of electrical resistivity well logging methods. More specifically, the disclosure is related to a method and apparatus for providing collocated coils and measuring cross-component magnetic fields in a downhole resistivity tool.
2. Description of the Related Art
Electromagnetic induction and wave propagation logging tools are commonly used for determination of electrical properties of formations surrounding a borehole. These logging tools give measurements of apparent resistivity (or conductivity) of the formation that, when properly interpreted, reasonably determine the petrophysical properties of the formation and the fluids therein.
A limitation to the electromagnetic induction resistivity well logging instruments known in the art is that they typically include transmitter coils and receiver coils wound so that the magnetic moments of these coils are substantially parallel only to the axis of the instrument. Eddy currents are induced in the earth formations from the magnetic field generated by the transmitter coil, and in the induction instruments known in the art. These eddy currents tend to flow in ground loops which are substantially perpendicular to the axis of the instrument. Voltages are then induced in the receiver coils related to the magnitude of the eddy currents. Certain earth formations, however, consist of thin layers of electrically conductive materials interleaved with thin layers of substantially non-conductive material. The response of the typical electromagnetic induction resistivity well logging instrument will be largely dependent on the conductivity of the conductive layers when the layers are substantially parallel to the flow path of the eddy currents. The substantially non-conductive layers will contribute only a small amount to the overall response of the instrument and therefore their presence will typically be masked by the presence of the conductive layers. The non-conductive layers, however, are those layers which are typically hydrocarbon-bearing and are of the most interest to the instrument user. Some earth formations which might be of commercial interest therefore may be overlooked by interpreting a well log made using the electromagnetic induction resistivity well logging instruments known in the art.
FIG. 2 shows a configuration of transmitter coils and receiver coils in an embodiment of the 3Dexplorer (3DEX®) induction logging instrument of Baker Hughes Incorporated. Three transmitters 201, 203, and 205 that are referred to as the Tx, Tz, and Ty transmitters are placed with their normals substantially orthogonal to each other, in the order shown. The three transmitters induce magnetic fields in three spatial directions. The subscripts (x, y, z) indicate an orthogonal system substantially defined by the directions of the normals to the transmitters. The z-axis is chosen to be substantially parallel to the longitudinal axis of the tool, while the x-axis and y-axis are mutually perpendicular directions lying in the plane transverse to the longitudinal axis. Also on the tool are receivers 211, 213, and 215, referred to as the Rx, Rz, and Ry receivers, aligned along the orthogonal system defined by the transmitter normals. Rx, Rz, and Ry are responsible for measuring the corresponding magnetic fields Hxx, Hzz, and Hyy. In this nominalization of the magnetic fields, the first index indicates the direction of the transmitter and the second index indicates the direction of the receiver.
One challenge associated with the logging instrument of FIG. 2 is that since each receiver and each transmitter are located at different axial positions, each transmitter-receiver combination has its own transmitter-receiver distance and records resistivity over different areas. Reducing these anomalies leads to improved resultant induction logs. U.S. Provisional Patent Application Ser. No. 61/020,637 of Wang et al., filed on Jan. 11, 2008, discloses collocated antenna arrangements. The teachings of Wang show the use of multiple sets of grooves in which the antenna coils are placed. This may increase the design complexity and may weaken the tool. The present disclosure provides embodiments for reducing these anomalies without using grooves.